Peak-limiting of audio signals is a very common practice is the audio industry throughout the process of recording, mixing, mastering, production, and distribution of audio content. Unprocessed audio content has a wide dynamic range, while all the common distribution media for audio content available today, including radio broadcasting, internet broadcasting, audio CD, DVD, and more, have a limited dynamic range. Further more, production requirements, and different listening-environment conditions, often require limiting the dynamic range of audio content.
Audio peak-limiters reduce the dynamic range of the audio signal, more specifically, peak-limiters reduce the dynamic range by ensuring the signal will not exceed a certain threshold, while maximizing the RMS of the resulted audio signal, and minimizing audible distortions.
Audio peak-limiters are very common in the audio industry, and are available for decades, ranging from the simplest analog audio ‘clipper’ to sophisticated digital-domain peak limiters. The technology of peak-limiters has improved since its beginning, allowing for better and deeper peak-limiting, with fewer unwanted audible artifacts. Since 1990 peak-limiters became a must tool in every commercial music production and radio broadcasting, as they were used to maximize the RMS output possible for the given limitations of the distribution media. As a result, there is constantly a strong demand to further improve the technology and achieve deeper peak-limiting, with fewer unwanted audible artifacts.
Smooth Attenuation Signal
The design goal for a peak limiter is to prevent the signal from ever exceeding a given threshold, while maximizing the output signal RMS, and minimizing unwanted audible side effects. While simple signal clipping can achieve peak-limiting, it has severe unacceptable audible distortions. To avoid these severe distortions typical to signal clipping, peak limiters apply a smooth attenuation signal to the audio signal. The smooth attenuation signal is derived such that peak-limiting is achieved, while the control signal is smooth enough to minimize unwanted audible distortions. Thus, deriving the smooth attenuation signal is an essential part of a good peak limiter. FIG. 1A to C show the difference between a clipped signal and a peak-limited signal, in this example the peak-limited signal uses look-ahead smoothing. In FIG. 1.A. the original sine wave is shown. In FIG. 1.B. two control signals are shown, one that will result in clipping, and a smoothed version of it. In FIG. 1.C. the resulting clipped and peak-limited signals are shown.
Prior art techniques to derive the smooth attenuation signal include proper selection of the attack and release attenuation signal smoothing time constants, look-ahead techniques, and the peak-limiter's input-output curve shape. A peak limiter should have very fast response for signal attacks; otherwise peak-limiting will not be achieved. One prior art technique for increasing the attack time while achieving peak-limiting, is the use of look-ahead. Look-ahead peak limiters use delay lines on the input signal, so that they can anticipate the peaks before they reach the output. By aligning the attack smoothing to the look-ahead time, improved peak-limiting can be achieved, with slower attack times. Typical attack time constants used in peak limiters are instant attack when no look-ahead is used, and few milliseconds when look-ahead is used. The release smoothing of a peak limiter has no effect on the peak-limiting itself, rather on the RMS of the resulting peak limited signal. Longer release times will sound less distorted, but will also have lesser RMS, because signal attenuation stays for longer times after the peaks. In order to optimize the tradeoff between RMS and amount of perceived distortions, some prior art peak limiters use adaptive release time technologies, where the release time is dynamically adjusted according to the signal being processed. Typical release time constants used in peak limiters are between few milliseconds to several hundred milliseconds, depending on the type of signal being limited, and the amount of limiting applied. It is to be noted that release time constants are typically much longer than attack time constants.
Inter-Modulation Distortions in Prior Art Peak Limiters
While considerably reducing audible distortions through proper attenuation signal smoothing, prior art wide band peak limiters are subjected to inter-modulation distortions between different instruments and different frequency bands in the audio mix.
Multi Bands Peak Limiters
One prior art technique to reduce inter modulations is multi bands peak limiter. In a multi bands peak limiter the input audio signal is split into several frequency bands, and each band is limited individually before mixing them back. Usually another wide band peak limiter is applied to the signal after mixing to ensure precise peak-limiting. This approach is sub-optimal since it either peak limits each band separately before mixing while ignoring information from the other bands (in the individual bands peak limiters), or peak limits the signal after mixing, while ignoring information from the individual bands (in the peak limiter after mixing).
Multi Tracks Peak Limiters
Similarly another common prior art technique in multi track audio mixing, is to peak limit each audio track individually before mixing, and to apply yet another peak limiter to the mixed signal. This approach is also sub-optimal for similar reasons to prior art multi bands peak limiters.
In the prior art invention described in U.S. Pat. No. 6,501,717 both the signals of the tracks before mixing and the prediction of the resulted mix are analyzed in conjunction, and attenuation is applies to each track before mixing so as to prevent the mix from overflowing. U.S. Pat. No. 6,501,717 concerns Level detectors that detect levels of digital audio signals of individual channels. A controller predicts whether or not an overflow will take place corresponding to detected signals. When the controller predicts an occurrence of an overflow, attenuators attenuate a signal level of at least one channel. In addition, when variable length delaying devices vary the phases of signals, an overflow can be prevented.
While this system offers a potential improvement over its prior art, it can only achieve sub optimal results for peak-limiting. One limitation of the '717 patent is that it does not relate to the effects of the smooth attenuation signal, which is essential for high quality peak-limiting. As shown in the '717 patent, this system relates only to the signals before mixing, but not to the attenuated signals, or to the smooth attenuation gain signals themselves. In the context of a peak limiter, failing to relate to the prediction of the smooth attenuation gain will lead to suboptimal results. This can be understood by noticing that if at some instant a certain track should anyhow be attenuated, regardless of its value, due to attenuation signal smoothing considerations, than attenuation of other tracks may be relieved proportionally, while still achieving peak-limiting of the mixed signal. Thus failing to relate to the attenuation imposed by the smoothing considerations will lead to sub optimal (too much) attenuation overall. Note that the '717 patent discloses that the level attenuation process is of the order of several seconds. For certain applications such as peak limiting, much shorter time constants should be used, in order to maximize the RMS of the peak limited signal, see for example the L1-UltraMaximizer and L2-UltraMaximizer products commercially available from Waves Audio Ltd.
U.S. Pat. No. 5,402,500 discloses an improved audio mixing system. The system is not limited to one type of microphone and does not depend on separate sensing microphones to determine the background noise level. In addition, the active signal is not amplitude modulated by extraneous noise inputted through inactive microphones. The mixing system divides the system gain between a plurality of input channels. The mixing system periodically samples the channels and determines which channel is the dominant channel. In response to this determination, the gain of the dominant channel is increased and that of the other channels decreased.
There is a need in the art for improved peak-limiting, while still processing the individual tracks before being mixed.
Terms and Definitions
There follows a list of known terms. The definitions are provided for clarity only and are by no means binding    Clipping includes: Clipping a signal S(t) to a threshold T can be formulated as follows: CS(t)=min(T, max(-T,S(t))). Normally, clipping of audio signals that exceed T by more than 1 or 2 dBs results in unacceptable distortions for high quality audio applications.    Peak-limiting includes: Peak-limiting a signal S(t) to a threshold T is done by attenuating the signal S(t), locally around points where its absolute value exceeds a threshold T, such that the absolute value of the resulting signal will not exceed T. Clipping is one way to implement peak-limiting, but in general peak limiting refers to more sophisticated processing applying smooth attenuation, that significantly reduces distortions compared with simple clipping.    Peak limiter includes: a signal processing device or algorithm that applies peak-limiting to its input signal, to provide a peak limited output signal.    Smooth attenuation includes: a common implementation of a peak limiter is to compute a smooth attenuation signal, and multiply it by the input signal to produce the peak limited output. In general, the smooth attenuation signal will be <1.0 around locations where the input signal exceeds the threshold. Smoother attenuation signals will keep being <1.0 further away from the locations where the input signal exceeds the threshold.    Audio Track includes: reference to mono audio signal, but also to the more general case of multi-channels audio track. A multi-channels audio track may be a stereo track, consisting of two channels, or a surround track, consisting of 5.1 or more channels.    Mixing includes: Summing of multiple audio signals together.    Multi Channels Mixing includes: mixing of multiple audio tracks, where each track consists of multiple channels and respective channels from each track are mixed to one respective channel of the output. Multi channels mixing differs from independent multiple monophonic mixers in that the amplitude relations between channels of the same input track should be maintained also in the output mix.    Down Mixing includes: down mixing is common when there is a need to play back multi-channels audio such as stereo or surround, trough a system with fewer output channels. For example, playing 5.1 surround recording through a stereo system. In the context of this invention reference is made to the fact that information from several channels has to be mixed in order to form the fewer output channels, and thus there is a risk of overflow in the output.    Frequency Bands Splitting includes: splitting the audio signal into multiple frequency bands, process them and mix back to form the output signal.